Due to their large F-number, contemporary microscope eyepiece lenses generate little spherical aberration at the image plane and require little aberration correction. Consequently, many conventional eyepiece lenses consist generally of lenses having spherical surfaces.
It is desirable, however, to widen the field of view of the typical microscope eyepiece lens. Also, to enable the user to look through the eyepiece lens while wearing eyeglasses, it is desirable to increase the eye relief (i.e., the distance between the eyepiece lens and the eye point). As the field of view and the eye relief of an eyepiece lens are increased, distortion and spherical aberration of the pupil tend to rapidly increase, posing the need to use aspherical lenses to correct these aberrations. Eyepiece lenses of this type are disclosed, for example, in Japanese laid-open patent document nos. SHO 62-255914, HEI 5-215974, HEI 7-270688, HEI 7-281108, and HEI 8-43749.
Aberration correction in eyepiece lenses requires simultaneous correction of both: (1) aberrations, at the image plane, in the image formed by rays traveling from the entrance pupil (the so-called "inherent aberrations" of the eyepiece lens), and (2) aberrations at the pupil position, as would be formed by light traveling from the image plane (the so-called "pupil aberrations"). Inherent aberrations and pupil aberrations are not completely independent, however, and cannot be independently corrected. Thus, it has been impossible to sufficiently correct these aberrations with an eyepiece lens comprising only spherical surfaces.
Even with conventional eyepiece lenses containing aspherical lenses, while some aberrations have been well corrected, other aberrations are often insufficiently corrected, or have even increased relative to eyepiece lenses without aspherical surfaces.
As objective lenses have been developed having increased numerical aperture, demand has increased for such eyepiece lenses having, in addition, improved resolution at the edge of the field of view. With conventional eyepiece lenses with aspherical surfaces, attempts to correct coma sufficiently to obtain high resolution to the edge of the field of view have been unsuccessful.
In addition, when changing objective lenses on the microscope, or when a relay lens or the like is used between the objective lens and the eyepiece lens, the position of the entrance pupil of the eyepiece lens typically changes. If the coma of the eyepiece lens is not adequately corrected, astigmatism will change significantly with changes in the entrance pupil position, which deteriorates resolution at the edge of the field of view.